Memcpy optimisation fun with overlapping pointers

memcpy.md

This demos how memcpy can go badly wrong when used to remove an element of an array by shifting content left.

#include <assert.h>
#include <stdbool.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

struct foo {
  int a;
  int b;
};

int main(int argc, const char **argv) {
  char data[16];
  for (int i = 0; i < sizeof(data); i++) {
          data[i] = (char)i;
  }

  struct foo *dst = malloc(sizeof(struct foo) * sizeof(data));

  for (int i = 0; i < sizeof(data); i++) {
    dst[i].a = i;
    dst[i].b = i;
  }

  // removes zeroeth element
  memcpy(&dst[0], &dst[1], sizeof(struct foo) * (sizeof(data) - 1));

  bool bad = false;
  for (int i = 0; i < (sizeof(data) - 1); i++) {
    fprintf(stderr, "(%d %d)", dst[i].a, dst[i].b);
    if (dst[i].a != i + 1) {
            fprintf(stderr, " expected (%d %d)", i+1, i+1);
            bad = true;
    }
    fprintf(stderr, "\n");
  }
  printf("\n");
  assert (!bad);
  return 0;
}

Compile with clang without optimisation and run

clang memcpy.c -o memcpy
./memcpy
(1 1)
(2 2)
(3 3)
(4 4)
(5 5)
(6 6)
(7 7)
(8 8)
(9 9)
(10 10)
(11 11)
(12 12)
(13 13)
(14 14)
(15 15)

So far so good...

Now try with -O2

clang memcpy.c -O2 -o memcpy
./memcpy
(1 1)
(2 2)
(3 3)
(4 4)
(5 5)
(6 6)
(7 7)
(9 9) expected (8 8)
(9 9)
(10 10)
(11 11)
(12 12)
(13 13)
(14 14)
(15 15)

Assertion failed: (!bad), function main, file memcpy.c, line 38.
[1]    84171 abort      ./memcpy

Tested on

# clang -v
Apple clang version 17.0.0 (clang-1700.4.4.1)
Target: arm64-apple-darwin25.1.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/bin
# sysctl -n machdep.cpu.brand_string
Apple M3 Ultra